Steam generators in pressurized water nuclear reactors generally comprise a very thick tubular plate through which extend holes in which the tubes of the tube nest of the generator are fixed by crimping. The tubular plate is in contact on one side with the pressurized water constituting the primary fluid and on the other side with the feed water, which is heated and vafluid.
When a tube of the nest of tubes of the steam generator is damaged, it is often necessary to extract it from the tubular plate and to close airtightly the hole passing through the tubular plate from which the tube has been extracted. It is in fact necessary to maintain complete separation between the primary fluid and the secondary fluid.
Various techniques have been proposed for closing the holes in a tubular plate in a steam generator which have been left open through extraction of tubes from the nest of tubes.
One of the techniques used consists in placing a stopper directly in the hole in the tubular plate. However, a stopper of this kind does not cover the hole in the tubular plate over its entire length, so that the tubular plate of low-alloy steel is exposed to corrosion by the secondary fluid in the interior of the hole in the uncovered zone. In addition, there is a risk that corrosive products may accumulate in the hole, above the stopper.
In order to avoid these disadvantages, it has therefore been proposed to fit a glove finger covering the entire surface of the hole through the tubular plate. A glove finger of this kind consists of a tube of a length at least equal to the thickness of the tubular plate and closed at one end by a welded end piece. The glove finger is fastened by expansion in the tubular plate over its entire length, the fluid-tight end piece joined by welding being disposed on the secondary side. In order to obviate the need to inspect the weld joint of the end piece periodically, the stresses which act on this weld joint are reduced by subjecting this weld joint and the end piece to equal pressures by forming a communication hole in the side wall of the glove finger. It is therefore necessary to achieve airtightness of the glove finger by means of a stopper, which is placed at the entry of the hole in the tubular plate and of the glove finger, on the primary side.
This technique, which gives satisfactory results in respect of the corrosion resistance of the tubular plate, nevertheless has the disadvantage of entailing a delicate operation and of needing a relatively long intervention time, on the part of the operators, inside the primary part of the steam generator. This means that the operators are exposed to radiation for a relatively long period of time.